1. Field of the Invention
The present invention generally relates to a multiplex transmission system. More particularly, the invention relates to a multiplex transmission system of the type in which a plurality of units of electrical equipment and appliances disposed at various points of a vehicle, such as an air conditioner, a cruising controller, an intermittently operating wiper, and a variety of lamps, are connected loop-like by at least one wiring harness and besides the signal transmission and reception thereamong are effected in a multiplex manner through the wiring harness.
2. Description of Relevant Art
Recently, with an increase in the number of electrical equipment and appliances to be disposed at various points of a vehicle, the wiring harness for interconnecting such equipment and appliances tends to have an increased number of wires and hence to be enlarged in size, thus lowering the design feasibility, productivity, fittability, and maintainability of the wiring harness, while raising indirect costs such as for control and storage. In conjunction with such problems, there is a growing tendency to employ a multiplex transmission system in which a plurality of multiplex harness control devices (hereinafter called "nodes") are connected loop-like by at least one wiring harness and the signal transmission and reception thereamong is effected in a multiplex manner. Some examples of such multiplex transmission system for vehicles are disclosed in "Jidosha Gijutsu (Automobile Technology)" Vol. 38, No. 2, published 1984, at pages 215 to 221.
This reference includes comments on a number of types of multiple transmission system, as follows.
According to the reference, multiplex transmission systems are classified into several types by the system of multiplexing, control, communication, and synchronization.
As multiplexing systems, there are enumerated a time division system and a wavelength division system, besides others; the former being suitable for multiplexing those transmission systems in which a wiring harness is employed as a signal transmission line, and the latter, for those in which an optical fiber is employed as a signal transmission line.
As control systems, typically there are two: a centralized control system in which principally a single node is responsible for the control of an entire system, and a distributed control system in which the control of a system is equally shared among a plurality of nodes. However, as a hybrid of typical systems, there may well exist a variety of control systems that are unable to be fairly classified into either of the typical two categories.
As communication systems, generally there are two: a parallel line system in which a signal of address and a signal of detection and control data are transmitted through separated lines, and a serial line system in which such signals are transmitted through a single line. In either system, there is additionally provided a power line with which respective nodes are connected loop-like.
The serial line system usually appears as either a message system in which an address signal and a detection and control data signal are combined to be transmitted as a single message, or a time-slot system in which an address signal is omitted while transmitting a series of detection and control data signals in a predetermined sequence.
As synchronization systems, there are employed two: an internal clock system in which respective nodes have clocks thereof, and an external clock system in which a clock signal is generated at a particular node and supplied therefrom to other nodes. In the latter system, the particular that clock generator is adapted to generate and supply the clock signal is used to govern the other nodes. Thus, this system may well be regarded as a variation of the centralized control system.
In this respect, in the centralized system, an entire system may be shut down with troubles at a master node. As a control system for the multiplex transmission system, therefore, the distributed control system is preferred to be employed, though in this case there is a desideratum to be attained by devising a communication system to avoid confusion of signal transmission among respective nodes.
Incidentally, when performing control of a load as an electrical equipment or appliance, it sometimes becomes necessary to execute a logical process in addition to on-off control of a single switch. Exemplarily, in the case of a vehicle, there are adduced reasons such as that some loads are required to have their patterns of action selectable depending on various positions of an ignition switch, such as "on", "off", "park", and "lock" positions and some loads such as a tail lamp are needed to be operable for on-off control from a plurality of switches such as a tail switch and a parking switch.
For such reasons, in general, in a time-division multiplex transmission system according to a distributed control system, each node is constituted with: (1) a combination of a transmitter and a receiver connected both to a wiring harness; (2) a detection circuit such as a sensor or a switch put under control of the node; (3) a drive circuit such as a relay or a power switch for starting a load such as a pump or a lamp put under control of the node; (4) a logic circuit connected to the detection circuit and the drive circuit; (5) a combination of a multiplexer and a demultiplexer for interconnecting the logic circuit with the transmitter and the receiver; and (6) a control circuit connected to, to cooperate with, the multiplexer and the demultiplexer and adapted for signal transmission to and reception from the wiring harness.
In the accompanying drawings, FIG. 6 is a schematic representation of a time-division multiplex transmission system for vehicles of such a type as suggested from the level of art described.
In the FIG. 6, designated by reference character 200 is the multiplex transmission system, which employs a centralized control system as the control system, a message type serial line system as the communication system, and an internal clock system as the synchronization system.
The multiplex transmission system 200 is constituted with n ordinary nodes 200-1 to 200-n each respectively having such circuitries as (1)-(6) above, and a bidirectional or bilateral signal transmission path (wiring harness) consisting of a transmission line 211 with which the nodes 201 to 205 are connected loop-like.
The node 200-1 is now supposed to be a master node adapted to supply a polling signal to the remaining nodes 200-2 to 200-n.
In the multiplex transmission system 200, from the respective nodes 200-1 to 200-n various signals thereof are sequentially transmitted to the signal transmission line 211, in a controlled manner, which sequence of signal transmission is considered to be of such a system as shown in FIG. 7.
In this respect, each of the nodes 200-1 to 200-n shall be provided with an internal clock for measuring a time base Tb employed for timing the signal transmission, and adapted to transmit, of any data, 4 bits within the time interval of time base Tb. In general, data length is given in terms of a unit of 8 bits, which means each unit length of data is transmitted within a period of 2.multidot.Tb.
A situation is now considered, where, while a series of polling signals P.sub.i (i=2 to n) are sent from the master node 200 sequentially to the nodes 200-2 to 200-n, only the node 200-2 has no data signals to be then transmitted and will not transmit any data signal.
Incidentally, to avoid complicatedness, in FIG. 7, there are shown no more than those signals transmitted from nodes 200-1, 200-2, 200-3, and 200-4.
From the master node 200-1, a polling signal P.sub.2 appointing the node 200-2 (of which signals are not shown) is first transmitted. The polling signal P.sub.2 is a signal of 1 byte to be transmitted within the period 2.multidot.Tb.
After having transmitted the polling signal P.sub.2, the master node 200-1 monitors, for a reception monitoring time Tm, whether or not signal transmission is performed from the node 200-2.
Such monitoring time Tm may preferably be determined to be the time base Tb times 2m (where m is an arbitrary integer, such that m.gtoreq.2), in consideration of the necessary time till initiation of signal transmission from voluntary node 200-i to be appointed by a polling signal P.sub.i. In the present case, the monitoring time Tm shall have such a relation to the time base Tb, that: Tm=4.multidot.Tb, while its length is measured by the internal clock of the master node 200-1.
When the monitoring time Tm has elapsed, the master node 200-1 judges that the node 200-2 will not perform signal transmission, and transmits signals including its own address a.sub.1 and necessary data d.
The signals a.sub.1 and d are supposed to be of a 1-byte length, respectively. Also the signals a.sub.1 and d are transmitted, depending on the internal clock of the master node 200-1. The other nodes 200-2 to 200-n recieve the signals a.sub.1 and d, to thereby execute their processes, as necessary.
After lapse of a monitoring time Tm' from transmission of the signals a.sub.1 and d, the master node 200-1 transmits a polling signal P.sub.3 identifying or appointing the node 200-3.
The node 200-3 receives the polling signal P.sub.3 and, after lapse of a transmission monitoring time tm at the slave node side, tries to transmit signals of own address a.sub.3 and necessary data d.
However, at the node 200-3, of which internal clock is not synchronized with that of the master node 200-1, an extra time Tw.sub.1 is further consumed from the time point where the monitoring time tm has elapsed, before the node 200-3 transmits the signals of own address a.sub.3 and necessary data d.
The signals a.sub.3 and d are supposed to be of a 1-byte length, respectively. Also the signals a.sub.3 and d are transmitted, depending on the internal clock of the node 200-3.
The master node 200-1 receives the signals a.sub.3 and d, and executes necessary processes. The time required for such process generally is extremely short, when compared with the time base Tb.
After lapse of a monitoring time interval Tm" from reception of the signals a.sub.3 and d, the master node 200-1 tries to transmit a polling signal P.sub.4 appointing the node 200-4.
However, at the master node 200-1, of which internal clock is not synchronized with that of the node 200-4, an extra time Tw.sub.2 is further consumed from the time point where the monitoring time Tm" has elapsed, before actual transmission of the polling signal P.sub.4.
Here, it is supposed that the internal clock of the node 200-4 happens to be synchronized with that of the master node 200-1.
Therefore, the node 200-4 is permitted to transmit signals of own address a.sub.4 and necessary data d after lapse of the transmission monitoring time tm at the slave node side from reception of the polling signal P.sub.4.
Like this, in the multiplex transmission system 200, the transmission and reception of signal among the respective nodes 200-1 to 200-n are performed in a cyclic manner, on the basis of polling signal P.sub.i from the master node 200-1.
In the multiplex transmission system 200, however, on the signal transmission line 211 there are transmitted, in addition to signals of own address a.sub.i and data d from any of the nodes 200-1 to 200-n, the polling signal P.sub.i from the master node. As a result, the signal density of the signal transmission line 211 is relatively low.
In this respect, the signal transmission system 200 may advantageously be modified such that, depending on address signal a.sub.i from a certain node 200-i, the node 200-i+l next to come for signal transmission is determined, while the order of signal transmission of the respective nodes 200-1 to 200-n is to be predetermined.
According to such modification, the polling signal P.sub.i becomes unnecessary, so that the signal density on the signal transmission line 211 can be all the more raised. Such modification may well be regarded as an introduction of the distributed control system.
However, even when such modification is applied to the multiplex transmission system 200, if the internal clocks of the nodes 200-1 to 200-n are not synchronized thereamong, there arises the problem that, upon signal transmission at the respective nodes 200-1 to 200-n, such time intervals as the extra times Tw.sub.1 and Tw.sub.2 of FIG. 7 are needed to be consumed again.
Moreover, in the multiplex transmission system 200, the order or time point of signal transmission of respective slave nodes 200-2 to 200-n is determined by the polling signal P.sub.i transmitted from the master node 200-1, thus accompanying such a problem that the entirety of the system 200 may malfunction with the polling signal P.sub.i kept from being transmitted in the event of malfunction of the master node 200-1. Even in the modificatoin above, such problem may occur if any node is caused to malfunction.
The present invention has been achieved to effectively solve such problems of conventional multiplex transmission systems of the type described.